Dew-point indicator and controller



Dec. 30, 1941. R. B. P. CRAWFORD 2,268,442

DEW-POINT INDICATOR AND CONTROLLER Filed March 10, 1938 3m ntor ROW13.1QC/wwJ-f/04d/ 1;

Gttorneg Patented Dec. 30, 1941 DEW-POINT INDICATOR AND CONTROLLERRobert B. P. Crawford, Miami, Fla., assignorto Minneapolis-HoneywellRegulator Company, Minneapolis, Minn., a corporation of DelawareApplication March 10, 1938, Serial No. 195,178

11 Claims.

This invention relates to a simple and accurate method for obtaining anindication of the vapor pressure or dew-point of air within a spacebeing conditioned and for controlling the humidity of the air in saidspace.

In carrying out my invention, I make use of the fact that certainmaterials such as lithium iodide have a very strong afllnity for watervapor and will change their moisture content materially with variationsin the vapor pressure of the air affecting them. The chemical is spreadover a large superficial area and the air whose moisture content is tobe sensed is circulated over the same. The weight of the chemical willchange in accordance with variations in the moisture content of the air,and this change is utilized to give an indication of the vapor pressureor the dewpoint and to exercise a control function. Means are alsoprovided to maintain the air passing in contact with the chemical at asubstantially fixed temperature so that a true indication of the vaporpressure or dew-point of the air may be obtained even though thetemperature of the air in the space may not remain constant.

It is an object of my invention to provide a simple apparatus foraccurately indicating the vapor pressure or dew-point of air.

Another object of my invention is to provide a simple apparatus foraccurately controlling a suitable conditioning means in accordance withthe vapor pressure or dew-point of the air to be conditioned.

Another object is the provision of means utilizing the aflinity ofcertain materials for water vapor for accurately controlling andindicating the vapor pressure or dew-point of the air in a space to beconditioned.

Other objects will become apparent upon a study of the specification andclaims in connection with the accompanying drawing in which:

Figure 1 is a schematic view of one form of my invention, certain partsbeing illustrated in cross section, and

Figure 2 is a view of a portion of Figure 1 illustrating a modificationthereof.

Referring more particularly to Figure 1, a casing I is provided throughwhich the air whose dew-point or vapor pressure is to be sensed iscirculated. Air is drawn into the casing through an inlet II and isexhausted through an outlet I2. A filter I3 which is of the closelymatted type is provided for removing all dirt particles from the air. Afan I4 driven by a motor I5 is located at the outlet I2 and serves tocause a circulation of air through the chamber.

A bracket IIi may be suitably secured to a wall of the casing I0, theupper portion ll of this bracket serving as a fulcrum for a lever I8.Carried at the left end of the lever I8 is a receptacle 20 made of anysuitable foraminous material and secured to the lever in any suitablemanner. The receptacle 20 contains a suitable porous or fibrous material2I which may be impregnated with a suitable chemical such as lithiumchloride or lithium iodide or any other suitable chemical having astrong aflinity for the water vapor in the air.

A spring 22 is connected to the lever I8 at the other side of thefulcrum I! from the end on which the receptacle 20 is mounted, thisspring being suitably connected to an adjusting screw 23 provided with asuitable operating handle 24 whereby the tension of the spring 22 may beadjusted. The end of the lever opposite that on which the container 20is mounted is suitably insulated from the rest of the lever byinsulating material 26 and is arranged to sweep across a resistanceelement 21. This resistance element may be connectedby means ofconductors 28 and 29 to terminals 30 and 3| of a motor 32. The end ofthe lever I8 may be connected by means of a conductor 33 to terminal 34of the motor 32. Line wires 35 and 36 are provided for supplying powerto the motor, these wires being connected to a suitable source of power(not shown). This motor may be a proportioning motor of the typeillustrated and described in Patent No. 2,028,110, issued to D. G.Taylor on January 14, 1936. An arm 38 is operated by the motor and itsposition is determined by the position of the right end of lever I8 withrespect to the resistance 21 in the manner described in the Taylorpatent.

Suitably mounted within the chamber I0 is a temperature responsivedevice 40 provided with a shielding means 4|. Thermostat 40 may comprisea bimetallic element 42 carrying an arm 43 and causing movement of thearm 43 in response to changes in temperature of the air in the casingI0. A fall in temperature in the easing I0 causes the arm 43 to movetoward the left and into engagement with a contact 45. A second fixedcontact 46 is engaged by the arm 43 when the temperature within thecasing I0 rises to a predetermined high value. Suitable magnets 4! and48 may be provided for giving a snap action to the arm 43 in a mannerWell known in the art to provide a suitable operating differential forthe thermostat. The contacts and 46 may be spaced sufficiently far apartso that when the arm is midway between the contacts it will be equallyattracted by the two magnets and will remain in mid position until thetemperature rises or falls to predetermined values.

Mounted in the upper portion of the casing I6 is a cooling coil 66through which a suitable cooling medium fiows, the flow of the coolingmedium being controlled by a valve 6|. This valve may include a valvestem 62 having an armature 66 connected thereto, this armature beingsurrounded by a solenoid 66. Energization of the solenoid 66 willattract the armature 66 and cause the valve to move to open position.Upon deenergization of the solenoid 66, the valve is moved to closedposition under the influence of a suitable biasing means (not shown).

Near the lower portion or the casing I6 is a heating element 66 which.for purposes of illustration, has been shown in the form of anelectrical heating element. It will be understood of course that anyother suitable form of heating means may be provided in place of theelectrical heating element. Lines 6! and 62 are provided for supplyingpower to the heating element 66, these lines being connected to asuitable source of power (not shown). The flow of current to the heatingelement 66 is controlled by a relay indicated at 66, this relaycomprising a relay coil 66, an armature 61, a relay arm 66, and a fixedcontact 66 cooperating with the arm 66. Upon energization of the coil 66the arm 66 is moved into engagement with the contact 66 and the heatingelement 66 is energized, current flowing from the line wire 6| throughconductor I6, switch arm 66, contact 66, conductor II, heating element66, and conductor I2 to the line wire 62. Upon deenergization of therelay coil 66, the arm 66 moves out of engagement with the contact 66under the infiuence'of gravity or any suitable biasing means (notshown), whereupon the heating element 66 is deenergized. Connected tothe line wires 6| and 62 is the high tension primary 16 or a step-downtransformer 16. The secondary 11 of the transformer 16 is provided forsupplying power to the relay coil 66 and the solenoid 66. When the arm66 of thermostat 62 is moved into engagement with the contact 66 therelay coil 66 is energized through the following circuit: from thesecondary 11 through conductor 66, bimetallic element 62, arm 66, comtact 66, conductor 6|, coil 66, and conductor 62 of the other side ofsecondary 'II. Energization of coil 66 causes arm 66 to move intoengagement with the contact 66, thus energizing the heating element 66.When the arm 66 movesout of engagement with contact 66 in response to arise in temperature in the chamber, the relay 66 is deenergized thuscausing the heater 66 to be deenergized. Upon a further rise intemperature the arm 66 will engage contact 66 and cause the energizationof solenoid 66 as follows: from secondary 11 of transformer I6 throughconductor 66, bimetallic element 62, arm 66, contact 66, conductor 66,solenoid 66, and conductor 66 to the other side of secondary 'II.Energization of solenoid 66 causes the opening of valve 6| whereuponcooling medium flows through the cooling coil 66. It will accordingly beseen that the tem: perature within the chamber I6 is maintained withinfixed limits by the thermostat 66 controlling the cooling element 66 andthe heating element 66.

Lines wires 66 and 66 are provided for supplying power to the motor I6fordriving the fan I6. In the circuit to the motor I6 is provided a coil66 of a relay6I. Relay 6| also includes an armature 62 and a relayarm66. Enersization of the relay coil 66 causes arm 66 tomove intoengagement with a fixed contact 66. Upon deenergization of the relaycoil 66, the arm 66 moves into engagement with a stop 66 under theinfluence of gravity or any suitable biasing means (not shown). A bell66 or any other suitable alarm device is controlled by the arm 66 andcontact 66. It will be apparent that when the relay coil 66 is energizedthe bell 66 will be energized through the following circuit: from theline wire 66 through conductors 66, 66, contact 66, arm 66, conductorI66, bell 66, and conductor IN to the line wire 66. The purpose of thisbell '66 is for giving an indication of the filter I6 becoming cloggedwith dirt and impeding the passage of air therethrough. When the filterI6 is clean and, permits a maximum flow of air through the filter, theload on the motor I6 will be at a minimum since the fan is operating ata low diflerential. The current through the coil 66 of relay 6| willbeinsuillcient when the load on the motor I6 is low to maintain the arm66 in engagement with contact 66. However, as the filter I I graduallybecomes clogged with dirt and other foreign matter there will be alesser flow of air through the filter and accordingly, since the fan I2is operating at a higher differential, the load on the motor II willincrease and hence the current through the coil 66 will increase. Whenthe current through the coil 66 has increased to a predetermined valueby reason of a large amount of dirt in the filter II the current throughthe coil 66 will be suflicient to move the arm 66 into engagement withthe contact 66 thus energizing the alarm device 66 and warning theattendant that the filter I6 is in need of being cleaned.

The motor 62 may be connected by means of a link I66 to a stem I66 of avalve I61 which may xcontrol the fiow through the pipe I66 of a suitablemedium for varying the humidity in the space which is being controlled.Since the particular form of apparatus for varying the humidity forms nopart of the present invention it has not been illustrated herein. Itwill be understood that a suitable refrigerant may flow through the pipeI66 to a suitable cooling coil in the air conditioning chamber, waterfor a suitable cooling spray may flow through the pipe I66, steam for areactivator for a chemical dehumidifying system may flow through thepipe I66 or any other suitable well known form of humidity changingapparatus may be controlled by the motor 62.

As the vapor pressure of the air passing through the chamber I6 varies,the material 2| in the receptacle 26 will absorb or give of! moistureaccording to whether the vapor pressure of the air is increasing ordecreasing. Thus as the vapor pressure of the air is increased a greateramount of moisture will be absorbed by the material 2| in the receptacle26 which causes the weight of the material to increase and the left endof lever I6 to move downwardly thus causing the right end of lever I6 tomove upwardly over resistance 21. Motor 62 will now operate in adirection to cause the apparatus controlled thereby to lower thehumidity in the space being controlled. Conversely as the vapor pressureof the air decreases the material in receptacle 26 will give of!moisture and lose weight whereupon the right end of lever I6 movesdownwardly under the influence of spring 22 and motor 62 is operated tocause the apparatus controlled thereby to increase the humidity of theair. It is essential that the temperature of the air passing through thechamber Ill be maintained at a substantially fixed value in order thatthe arm II will accurately control the motor 32 in accordance with thevapor pressure of the air. It the temperature in the space whosehumidity is being controlled should vary, the device 20 would notaccurately respond to the vapor pressure unless the temperature of theair around the receptacle 20 were maintained at a fixed value since theamount of moisture the material 2| absorbs will depend both upon thehumidity and the temperature of the air passing thereover.

A suitable indicating device may also be operated by the lever I8 togive a visual indication of the vapor pressure or dew-point of the air.This indicator may comprise alever H pivoted at III in a wall of thechamber III, one end of this lever being connected by a link H2 to thelever I8. The opposite end of the lever III is arranged to sweep over agraduated scale H3. This scale H3 may be graduated in terms of vaporpressure or dew-point or two scales for indicating both vapor pressureand dew-point, may, if desired, beprovided. As the receptacle 20increases or decreases in weight in response to changes in vaporpressure of the air, the right end of the lever H0 will move over thescale H3 and give an accurate indication of the vapor pressure ordew-point of the air. The adjusting screw 23 provides for adjustment ofthe device so that it will accurately control and indicate the vaporpressure of the air. Should, for any reason, the material in receptacle20 become contaminated it may be necessary to adjust the tension .ofspring 22 by the screw 23 to bring the device back into propercalibration. However, with filter I3 kept in place and maintained cleanthis last contingency should not arise under normal conditions.

The superficial area of the material 2I within the receptacle 20 shouldbe very large in order that the material will rapidly respond tovariations in the vapor pressure of the air.

In Figure 2 the alarm circuit is illustrated in connection with acentrifugal fan designated by the reference character H4 which may beprovided for causing a circulation of air through the chamber ID inplace of the fan I4. Whereas, the load on the driving motor for a screwor propeller type fan as illustrated in Figure 1 increases as the supplyof inlet air to the fan decreases by reason of the increase in thedifferential across the fan blades, a centrifugal fan acts in theopposite manner. In other words, when the supply of air to a centrifugalfan is diminished, the load on the fan driving means decreases.Accordingly when a centrifugal fan is used in place of the fan I4 inFigure 1 the load on the driving motor will decrease when the filter Hbecomes clogged in contrast to the increasing load on the motor I5 ofFigure 1 when a fan of the type shown in that figure is provided forcirculating air through the chamber ID.

The relay I9I of this figure may be a low voltage release relay of anysuitable type as for example of the type shown in Patent 2,101,548issued to C. G. Kronmiller on December'l, 1937. When the filter II isclean and the fan is drawing a maximum amount of air through the chamberI0 the load on the fan driving motor will be at a maximum so that amaximum amount of current flows through the relay coil i90, and

relay arm I93 is maintained in engagement with the fixed stop I95 and nocurrent flows through the alarm device I96. As the filter becomesclogged and the amount of air drawn into the casing decreases, the loadon the fan motor will decrease until the amount of current flowingthrough the coil I will be insufiicient'to maintain the arm I93 inengagement with the stop I95. The arm will now move under the influenceof gravity or any suitable biasing means into engagement with the fixedcontact I94 whereupon current will fiow through the alarm device I96 asfollows: from the line I88 through conductors I98, I99, contact I94, armI93, conductor 200, alarm device I96, and conductor 2M to the line I89.The operation of the alarm device will now indicate that the filter isin such a dirty condition that it should' be replaced or cleaned. Itwill thus be seen that the alarm device may be operated to indicate adirty condition of the filter with either a centrifugal fan or apropeller type fan by suitably positioning the stop 94 and contact 95 ofFigure 1.

It will now be seen that I have devised a rel atively simple butaccurate device for indicating vapor pressure or dew-point of air andfor exercising a control over suitable air conditioning apparatus tocontrol the'humidity of the air, the device responding very rapidly tochanges in the vapor pressure.

Having described the preferred form of my invention, many modificationsmay become apparent to those skilled in the art and I wish it to beunderstood therefore that my invention is limited only by the scope ofthe appended claims.

I claim as my invention:

1. In a device of the class described, a chamber, means for circulatingair .through said chamber, means for filtering the air before it enterssaid chamber, a quantity of a porous material impregnated with ahygroscopic substance located within said chamber, a control device 0D-erated in accordance with variations in weight of said porous materialoccasioned by varying degrees of humidity in the air passing throughsaid chamber, and means responsive to the load on said circulating meansfor indicating a dirty condition of said filtering means.

2. In a device of the class described, a chamber having an air inlet andan air outlet, filtering means in said air inlet, fan means for drawingair into said chamber through said filtering means, means operated inresponse to the load on said fan means for indicating a dirty conditionof said filtering means, and means responsive to the moisture content ofthe air passing through the chamber for indicating the moisture contentthereof.

3. In a device of the class described, a chamber having an air inlet andan air outlet, filtering means in said air inlet, fan means for drawingair into said chamber through said filtering means, means operated inresponse to the load on said fan means for indicating a dirty conditionof said filtering means, means for indicating the moisture content ofthe air, means for controlling the moisture content of the air, andmeans within said chamber responsive to the moisture content of the airin control of said last two mentioned means.

4. In a device of the character described, a chamber having an inlet andan outlet, means for drawing air through said inlet and exhausting itthrough said outlet, cooling means near the upper portion of saidchamber, heating means near the lower portion 01' said chamber,

temperature responsive means in said chamber in control of said heatingand cooling means, a ioraminous container within said chamber betweensaid heating and cooling means, said container containing materialhaving hygroscopic properties, and means controlled by the weight ofsaid material for indicating and controlling the moisture content of theair entering said chamber.

5. In a device of the character described, a chamber having an inlet andan outlet, means for drawing air through said inlet and exhausting itthrough said outlet, cooling means near the upper portion oi saidchamber, heating means near the lower portion of said chamber,temperature responsive means in said chamber in control of said heatingand cooling means, a toraminous container within said chamber betweensaid heating and cooling means, said container containing materialhaving hygroscopic properties, means controlled by the weight of saidmaterial for indicating and controlling the moisture content 01' the airentering said chamber, a filter in said inlet, and means responsive tothe load on the means for drawing air through said inlet for indicatinga dirty condition of said filter.

6. In a device of the character described, a chamber having an inlet andan outlet, means for drawing air through said inlet and exhausting itthrough said outlet, cooling means near the upper portion of saidchamber, heating means near the lower portion of said chamber,temperature responsive means in said chamber in control of said heatingand cooling means, a Ioraminous container within said chamber betweensaid heating and cooling means, said container containing materialhaving hygroscopic properties, filter means for removing impurities fromthe air entering said chamber, means controlled by the weight of saidmaterial for indicating the moisture content of the air entering saidchamber, and means for indicating a dirty condition 01! the filter,which dirty condition oi the filter reduces the air fiow through saidchamber and renders the means for indicating the moisture content of theair inefiective to indicate accurately.

7. In a device of the character described, a chamber having an inlet andan outlet, means for drawing air through said inlet and exhausting itthrough said outlet, cooling means near the upper portion of saidchamber, heating means near the lower portion of said chamber,temperature responsive means in said chamber in control of said heatingand cooling means, a foraminous container within said chamber betweensaid heating and cooling means, said container containing materialhaving hygroscopic properties. filter means for removing impurities fromthe air entering said chamber, means controlled by the weight 01' saidmaterial tor indicating and controlling the moisture contentof the airentering said chamber, and means for indicating a dirty condition oi thefilter, which dirty condition of the filter reduces the air flow throughsaid chamber and renders the moisture content indieating and controllingmeans ineflective to properly indicate and control the moisture content01' the air entering said chamber. v

8. In a device of the character described, a chamber, said chamberhaving an inlet and an outlet for facilitating the fiow 01' air throughsaid chamber, cooling means insaid chamber, heating means in saidchamber, means responsive to the temperature within said chamber forcontrolling said cooling means and heating means to maintain saidtemperature at a substantially constant value, means within said chamberhaving hygroscopic properties, and means movable in response tovariations in the weight of said hygroscopic means.

9. In a device oi the class described, a chamber having an air inlet andan air outlet, filtering means in said air inlet, Ian means for drawingair into said chamber through said filtering means, means for operatingsaid tan including a motor and an electrical circuit therefor, means forindicating the existence of a dirty filter, said indicating means beingresponsive to a predetermined variation of a characteristic 01' saidelectrical circuit for said in means and including means operated bysaid fan operating circuit in response to a predetermined load on saidfan.

10. In combination comprising, a filtering means" for air, a fan forcausing a fiow of air through said filtering means, an electric motordriving said ian, means for indicating a dirty condition 01' saidfiltering means, and means responsive to the amount of current drawn bysaid motor for operating said means for indicating a dirty condition 01'the filtering means.

11. In combination comprising, a filtering means, a fan for causing afiow of a gaseous medium through said filtering means, an electric motordriving said ian including an electrical circuit therefor, means forindicating a dirty condition of said filtering means, and meansresponsive to a predetermined variation of a characteristic of saidelectric circuit for operation 0! the fan driving motor for operatingsaid means for indicating a dirty condition 01' the filtering means.

ROBERT B. P. CRAWFORD.

